Process of preparing columbium and tantalum



United States Patent 2,994,603 PROCESS OF PREPARING COLUMBIUM AND TANTALUM Harry Greenberg and Raymond A. Foos, Cincinnati, Ohio, assignors to National Distillers and Chemical Corporation, New York, N.Y., a corporation of Virginia No Drawing. Filed June 20, 1958, Ser. No. 743,472 4 Claims. (Cl. 7584.4)

This invention relates to a new and improved process for the production of tantalum and columbium metals. More particularly, the invention pertains to the production of these metals by treatment of alkali metal tantalum and columbium fluorides.

In the prior art, tantalum and columbium have been recovered from their double salts, e.g. potassium tantalum fluoride, by electrolysis. This method is expensive because a hydriding and a crushing operation is required to convert the product to finely divided material. There also have been recent proposals to utilize chemical reduction processes on a commercial scale. In general, these processes have involved the use of such known re ducing agents as sodium, lithium, magnesium, aluminum, etc. Sodium has been found, for example, to be particularly effective. Chemical reduction processes have, however, certain problems connected therewith. For one thing, after the reaction between the potassium fiuotantalate and the reducing agent is initiated, a sudden generation of heat develops with the result that very high instantaneous temperatures occur in the reaction mixture. Hot spots develop in the bed which cause alloying of the tantalum or columbium with the walls ofthe reactor aswell as agglomeration of the tantalum or columbium metal. Leaching of the entrapped salt from the metal product recovered from such a reduction is very diflicult. In view of these runaway heats of reaction, it is necessary for the reduction to be carried out on a limited scale in order to maintain a controlled reaction. It is also virtually impossible in such chemical reduction processes to control particle size distribution. Thus, for example, the production of finely divided tantalum metal, Which has been found to be particularly useful in the electronics field for fabricating electrolytic capacitors cannot be carried out on a large scale by this method.

It is one object of this invention to provide a chemical reduction process which overcomes the difliculties encountered in the prior art processes. It is another object of this invention to provide a process wherein finely divided tantalum and columbium metals can be readily produced with control over the particle size distribution of the metal product. Other objects and advantages will become apparent from the ensuing description of the invention.

In accordance with the present invention, it has been found that these objects can be achieved by carrying out the reduction of the double salts of tantalum and colum bium with the aforementioned reducing agents in the presence of a finely divided alkali metal salt. More specifically, by conducting the reduction step in the presence of an inert alkali metal salt diluent it is possible to control the particle size distribution of the metal product, to avoid runaway temperatures from occurring in the reaction mixture and to produce high quality metal.

The double salts of tantalum and columbium which may be employed in the process of this invention have the following structural formula:

wherein R is an alkali metal such as potassium, sodium, lithium; M is either tantalum or columbium; and X 1s a halogen atom selected from the group consistmg of fluol a'tented Aug. 1, 1961 dium is the preferred reducing agent, although other known materials such as lithium, aluminum, potassium,-

etc., may also be employed. In general, the amount of reducing agent employed will be suflicient to stoichio-- metrically reduce the tantalum or columbium double salt to the respective metal. It will be understood, however, that the use of an excess of the reducing agent is also- The diluent within the scope of the present invention. tends to absorb any excess of reducing agent after comtrol is desired.

The alkali metal salts which are particularly useful for the present purposes are sodium and potassium halides. Examples of such compounds include sodium chloride,

potassium chloride, sodium bromide, potassium bromide,.

sodium iodide, potassium iodide, etc., or mixtures thereof. The chlorides of sodium and potassium are especially preferred. In general, the weight ratio of the alkali m'etali salt to the double salt of tantalum or columbium will be: 0.5:l.0 to 10:1, preferably about 1:1 to 2:1. It has: further been found that not all alkali metal halides are operable in the present invention. Thus, for example,

when potassium fluotantalate is reduced with sodium, lithium chloride cannot be employed, since it was found that lithium fluoride, which forms by an exchange reaction, was impossible to leach completely from the tantalum metal product.

As generally practiced, the process of this invention is carried out in a rather simple manner. The double salt of tantalum or columbium, the reducing agent and the alkali metal halide diluent are mixed together in a conventional reaction vessel. The resulting mixture is then heated to a temperature of about to 800 C., preferably 600 to 800 C., for about 2 to 8 hours to complete the reaction. If desired, the reaction mixture may be subjected to intermittent or continuous agitation during the course of the reaction. After the reaction is completed, the reaction vessel is cooled to room temperature and the reaction product mixture comprising the tantalum or columbium metal product and the various byproduct salts are recovered. The metal product is. usually obtained in a finely dispersed form throughout the salt mass; this mixture of metal and salt is commonly referred to as spalt. The metal may be recovered from the spalt by grinding the latter, and then subjecting the ground particles to conventional leaching operations to dissolve the salt by-products. Though the leaching step does not constitute an inventive feature of this process, it will be understood that leaching solutions such as water, dilute mineral acids, etc. may be employed for this purpose.

It will be understood that the exact manner of carrying out the reduction step is not limited to methods described above. It is possible, for example, to add the reducing agent in vapor, liquid or solid form. More specifically, the process may be initiated using alternate solid layers of the double salt and sodium pellets. Another method recently devised, involves adding solid or molten sodium slowly in increments to the reaction mixture. In order to achieve the results of this invention it is essential, however, to employ an inert alkali metal salt in the reduction step.

In order to more clearly describe the inventive process, the following examples are presented for illustrative purposes. Though these examples are limited to the preparation of tantalum metal, it will be understood that colum'bium, titanium, zirconium, hafnium, etc., may be prepared in a similar manner from their double salts.

Example I One mole of KgTaFq (392.1 gins.) was added to a stainless steel reactor provided with suitable stirrer, inlet and outlet gas sidearms for maintaining an inert atmosphere, a thermocouplewell, and a charging port for both solids and molten reactants. The reactor was surrounded by an electrically heated air furnace, and the temperature raised to about 350 C. with stirring to remove residual traces of Water vapor. The reactor'was maintained under an argon atmosphere until the termination of the run. After three hours, the reactor was cooled to about 100 C. and molten sodium (116 gins.) added without stirring. A 130 C. with stirring the reaction was initiated as indicated by the sudden generation of heat causing the temperature to rise to 600 C. This temperature was maintained for eight hours with occasional agitation. The reactor was then cooled to room temperature, and the spalt removed by chipping. The spalt was ground to a particle size of about 12 mesh, and leached with water to remove the water soluble salts. The powdered tantalum metal was 98.1% of the theoretical yield and had the following particle size distribution:

Mesh size: Product, percent +100 29.7 100 to 200 8.6 200 to 400 9.4 4OO 52.2

Example ll Mesh size: Product, percent +100 0.6 -100 to 200 9.6

Example III In this run the same operating conditions and apparatus were employed as in Example II, with the exception that KCl was used in place of NaCl. After eight hours of sintering, the tantalum recovered from the reaction product mixture had the following particle size distribution:

Product, Percent Mesh Size N 9.01 KC] Example IV Into a stainless steel reactor provided with an inert argon atmosphere, was charged 80 pounnds of K2THF7 with varying amounts of sodium chloride as shown in the table. The contents of the reactor were stirred and heated to 800 C. Molten sodium was added incrementally at the rate of 1 pound every three minutes until the 3% excess had been added. Sintering for 8 hours with stirring was eifected for each run. Upon cooling and removal from the reactor, followed by leaching, the particle size distribution of the isolated tantalum was as shown in the following table.

Particle Size Distribution, Mesh KzTaF N s01, Run No. lbs. lbs.

+30 30 to 100 to -325 Percent Percent Percent Percent 80 0 .9 18.6 18. 4 2.1 80 80 36. 6 17. 5 35. 5 10. 5 80 7. 5 22. 5 43. 0 27. 0

The data in the foregoing examples show that the particle size of the metal product may be readily control-led by utilizing an alkali metal halide salt diluent in the reaction mixture. As shown in, Example IV, the particle size distribution was somewhat afiected by utilizing a to the production of excessive fines.

' Though the present invention has been illustrated above in a number of specific embodiments, it will be understood that the invention may be obviously subject to variations and modifications without departing from its" broader aspects.

What is claimed is:

1. A method for preparing finely divided metal whichcomprises reacting a compound, having a structure represented by the formula RgMXq wherein R is an alkali metal, M is a metal selected from the group consisting of columb-ium and tantalum and X is a halogen atom selected from the group consisting of fluorine, chlorine and bromine, with an alkali metal reducing agent in the presence of an alkali metal halide selected from the group consisting of sodium and potassium chlorides, the weight ratio of said alkali metal halide to said compound being within the range of about 0.5: 1.0 to 10.0:l.0, at a temperature within the range of about 150 to 800 C., said alkali metal halide being initially present in the reaction mixture, and recovering said selected metal in finely divided form from the re.- sulting reaction product mixture including said alkali metal halide.

V 2. The method of claim 1 wherein R is potassium: and X is a fluorine atom.

3. The method of claim 1 wherein said alkali metal reducing agent is sodium.

4. The method of claim 1 wherein said alkali metal halide is sodium chloride.

References Cited in the file of this patent UNITED STATES PATENTS 2,827,371 Quin Mar. 18, -8

FOREIGN PATENTS 762,541 Great Britain Nov. 28, 1956 541,516 Canada May 28, 1957 791,121 Great Britain Feb. 26, 1958' These There is, nevertheless, still a tendency to pick up impurities and losses are high due 

1. A METHOD FOR PREPARING FINELY DIVIDED METAL WHICH COMPRISES REACTING A COMPOUND, HAVING A STRUCTURE REPRESENTED BY THE FORMULA 